[graemep]

> spray painting British military equipment.

Spraying paint down military jet engines rendering them inoperable until repaired, at a cos of millions of pounds.

> historically terrorists would actually need to commit acts that instil a sense terror in people to further their political objective

The legal definition of terrorism in the UK has for many years (at least all of the current century, I think a lot longer) included "serious damage to property":

https://www.cps.gov.uk/crime-info/terrorism

and I think causing many millions of pounds worth of damage is clearly serious.

I do not entirely agree with the definition (I particularly oppose making collecting information and disseminating publications terrorism) but it is what has long been accepted.